Physical layer structures and initial access schemes in an unsynchronized communication network

ABSTRACT

Physical layer structures and related access schemes for unsynchronized communication networks are provided. Access channel information, preferably including a common synchronization code associated with all transceiver stations in a communication network and a cell-specific synchronization code uniquely associated with one of the transceiver stations, is modulated onto at least one set of time-continuous signal components of a communication signal. In order to access the communication network, communication terminals search for the access channel information in one or more sets of time-continuous signal components and synchronization parameters are then determined based on a location of the access channel information in the sets of time-continuous signal components. Some embodiments of the invention provide for joint frame synchronization and coarse timing synchronization. In further embodiments, the communication signal also includes a scattered pilot channel onto which a portion of the access channel information, preferably the cell-specific synchronization code, is modulated. The pilot channels may then be re-used for initial access operations in addition to its conventional uses for such operations as channel estimation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 10/760,424, filedJan. 21, 2004, which claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application Ser. No. 60/441,105, filed on Jan. 21, 2003,both of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to unsynchronized communication networks, and inparticular to physical layer structures and access schemes for use insuch networks.

BACKGROUND OF THE INVENTION

In OFDM (Orthogonal Frequency Division Multiplexing) wirelesscommunication networks, data streams are typically transmitted inparallel using multiple orthogonal sub-carriers or tones within a singlechannel. The use of orthogonal sub-carriers allows the sub-carriers'spectra to overlap, thus achieving high spectrum efficiency. An OFDMsystem maps coded or modulated information symbols, QPSK (QuadraturePhase Shift Keying) or QAM (Quadrature Amplitude Modulation) symbols forinstance, to sub-carriers in the frequency domain, and then generates atime domain signal for transmission using such a transformationtechnique as IFFT (Inverse Fast Fourier Transform). At a receiver, atime-to-frequency transformation, such as an FFT (Fast FourierTransform), is used to convert a received time domain signal into thefrequency domain. In order to recover transmitted source symbolscorrectly, the receiver aligns an FFT window with a corresponding IFFTwindow used at the transmitter and compensates for any frequency offsetbetween the transmitter and the receiver.

Initial access to a communication network by a communication terminalinvolves a search operation to find available base stations andcommunication channels and a synchronization operation to synchronizethe terminal to a base station. Dedicated physical channels, such as aninitial access channel, a synchronization channel for timing andfrequency synchronization, and a pilot channel to assist in channelestimation for coherent detection have been used for these operations,but increase communication signal overhead.

Other known network access techniques involve the insertion of apreamble or some other synchronization signal by a transmitter, at apredetermined location in a communication signal, and detection of thesynchronization signal at a receiver. For synchronized communicationnetworks, preambles from multiple base stations in the communicationnetwork are transmitted at the same time. Orthogonal preambles supportefficient channel searching during initial access operations at areceiver. However, for asynchronous communication networks,synchronization signal transmissions from multiple base stations are notorthogonal, which degrades the performance of access algorithms based onsynchronization signals. A further issue with preamble-basedsynchronization signal insertion schemes is that payload management maynot be compatible with current standardized timing structures, such asthe UTRAN (UMTS (Universal Mobile Telecommunication System) TerrestrialRadio Access Network) timing structure, i.e. the 3GPP (3^(rd) GenerationPartnership Project) TTI or transmit-time-interval. Specifically, thepayload of a synchronization signal TTI would not be the same as thenormal TTI.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method includesmodulating access channel information onto at least one set oftime-continuous signal components of a communication signal that hasmultiple of signal components, each set of time-continuous signalcomponents having a respective common frequency, and transmitting thecommunication signal.

The access channel information preferably includes a commonsynchronization code associated with all transceiver stations in acommunication network and a cell-specific synchronization code uniquelyassociated with one of the transceiver stations. Both codes arepreferably modulated onto each of the at least one set oftime-continuous signal components.

The communication signal may also include a scattered pilot channel ontowhich a portion of the access channel information, preferably thecell-specific synchronization code, is also modulated.

In a preferred embodiment, the communication signal is an OFDM signal.Each set of time-continuous signal components includes signal componentscarried by respective sub-carriers in multiple OFDM symbols, and thescattered pilot channel is preferably pair-wise scattered ontosub-carriers having a common sub-carrier index in pairs of consecutiveOFDM symbols.

A method of accessing a communication network is provided in anotherembodiment of the invention. The method preferably includes receiving acommunication signal having a number of sets of time-continuous signalcomponents and searching for access channel information in at least onepredetermined set of time-continuous signal components. Synchronizationparameters are then determined based on a location of the access channelinformation in the at least one predetermined set of time-continuoussignal components.

When the access channel information includes a common synchronizationcode and a cell-specific synchronization code, searching includesrespective operations for searching for each code.

Searching for the common synchronization code preferably includessampling the received communication signal, performing a time domain tofrequency domain transformation using a transformation window startingat a start position to generate a frequency domain signal, extractingfrequency domain data corresponding to the at least one predeterminedset of time-continuous signal components from the frequency domainsignal within a window having a length of a predetermined period,correlating the extracted data with the common synchronization code,moving the predetermined period-length window by a predetermined stepsize until a starting position of the predetermined period-length windowhas been moved a distance of at least the predetermined period, andrepeating the extracting and correlating for each position of thepredetermined period-length window. Peak correlation values indicateoccurrences of the common synchronization code and therefore thelocation of the first symbol in each frame.

For each synchronization parameter determined on the basis of the commonsynchronization code, cell-specific searching preferably includesperforming the time domain to frequency domain transformation using thecoarse timing position estimate as the transformation start windowposition, extracting frequency domain data corresponding to the at leastone predetermined set of time-continuous signal components from thefrequency domain signal, starting from the candidate frame boundariesobtained from common synchronization code searching based framesynchronization step, correlating the extracted data with each of thecell-specific synchronization codes, and determining peak correlationvalues indicating occurrences of one of the cell-specificsynchronization codes.

A coarse timing position estimate is determined in some embodiments bymoving the transformation window by a transformation window step sizeuntil a starting position of the transformation window has been moved adistance of at least the symbol length, and for each position of thetransformation window, repeating the performing, extracting, correlatingwith the common synchronization code, moving the predeterminedperiod-length window, repeating the extracting and correlating, anddetermining peak correlation values.

A method according to a still further embodiment of the inventionincludes modulating a cell-specific synchronization code uniquelyassociated with one a base transceiver station in a communicationnetwork onto a scattered pilot channel carried by predetermined pilotchannel sub-carriers of a communication signal and transmitting thecommunication signal. At a receiving end, a communication terminalextracts data from the scattered pilot channel, correlates with thecell-specific synchronization code, and to do base transceiver stationidentification checking. It can also performs fine timing and frequencysynchronization operations based on the scattered pilots. Thecell-specific synchronization code may also be modulated onto one ormore sets of time-continuous signal components in the communicationsignal, preferably along with a common synchronization code associatedwith all base transceiver stations in the communication network.

In some embodiments of the invention, a computer-readable medium storesinstruction which, when executed by a processor, perform any of thesemethods.

According to yet another embodiment of the invention, a physical layerstructure for communication signals includes symbols having signalcomponents carried by respective sub-carriers and an initial accesschannel for carrying a synchronization code for use in synchronizationoperations at a communication terminal. The initial access channel ismapped to a time-continuous set of the signal components, thetime-continuous set of signal components including signal componentsfrom multiple symbols carried by at least one of the sub-carriers.

A base transceiver station in a communication network, according to afurther embodiment of the invention, includes a processor configured tomap a synchronization channel to a set of time-continuous signalcomponents in a communication signal and an output configured totransmit the communication signal.

In a related embodiment, a communication terminal includes an inputconfigured to receive a communication signal having signal componentscarried by respective sub-carriers and a processor configured to searchfor synchronization channel information in predetermined time-continuoussets of the signal components carried by respective ones of theplurality of sub-carriers and to determine synchronization parametersbased on a location of the synchronization channel information in thepredetermined time-continuous sets of the signal components.

Another embodiment of the present invention provides a communicationnetwork. Each of a number of base transceiver stations in the networkmodulates access channel information onto at least one set oftime-continuous signal components of a communication signal, each set oftime-continuous signal components having a respective common frequencyand transmits the communication signal to communication terminalsconfigured for operation in the communication network. The communicationterminals search for the access channel information in the plurality ofsets of time-continuous signal components and determine synchronizationparameters based on a location of the access channel information in theplurality of sets of time-continuous signal components.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of the specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying diagrams, in which:

FIG. 1 is a block diagram of a wireless communication network;

FIG. 2 illustrates a known physical layer structure for a synchronizedcommunication network;

FIG. 3 is a time-frequency representation of an example preamblestructure for a synchronized communication network;

FIG. 4 illustrates a physical layer structure for an unsynchronizedcommunication network;

FIG. 5 illustrates a physical layer structure in accordance with anembodiment of the invention;

FIG. 6 shows an initial access channel and scattered pilot channelsub-carrier pattern according to an embodiment of the invention on atime-frequency plane;

FIGS. 7( a) and 7(b) together form a table showing an example allocationof SSCs (Secondary Synchronization Codes) for secondary SCE(Synchronization Channel) sequences;

FIG. 8 is a block diagram showing a mapping between a synchronizationchannel and OFDM symbols in accordance with an embodiment of theinvention;

FIG. 9 is a block diagram showing a mapping between a synchronizationchannel and OFDM symbols in accordance with a further embodiment of theinvention;

FIG. 10 is a flow chart illustrating an initial access method accordingto an embodiment of the invention;

FIG. 11 is a flow chart showing an initial access method according toanother embodiment of the invention;

FIG. 12 is a block diagram of a communication terminal;

FIG. 13 is a block diagram of a synchronization code detector;

FIG. 14 is a plot of correlation value versus OFDM symbol indexillustrating an example joint frame synchronization simulation result;and

FIG. 15 is a plot of correlation peak value versus OFDM sample index forFFT window positions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a wireless communication network. Thecommunication network includes base transceiver stations (BTSs) 10, 12,14, which provide communication network coverage to respective coverageareas or “cells” 20, 22, 24. User equipment (UE) 16 is adapted tocommunicate with any of the BTSs 10, 12, 14 that are within its range,represented at 26.

Those skilled in the art will appreciate that the communication networkshown in FIG. 1 is intended solely for illustrative purposes, and that acommunication network may include further or different components thanthose explicitly shown in FIG. 1. For example, most communicationnetworks include more than three BTSs and provide communication servicesfor many UEs. Such communication networks are also normally connected toother types of networks, including landline telephone networks, forinstance. It should be further appreciated that BTS coverage areas andUE ranges are not normally hexagonal.

Each BTS 10, 12, 14 preferably includes a transceiver, or alternativelya separate transmitter and receiver, for sending communication signalsto and receiving communication signals from the UE 16 via an antennasystem. An antenna system at a BTS may include a single antenna or amultiple antennas, such as in an antenna array, for example. The BTSs10, 12, 14 may also communicate with each other, and with othercommunication stations or components, including components in othercommunication networks, through wireless or wired communication links.Communication functions of the BTSs may involve such operations asmodulation and demodulation, coding and decoding, filtering,amplification, and frequency conversion. These and possibly other signalprocessing operations are preferably performed in the BTSs by digitalsignal processors (DSPs) or general-purpose processors that executesignal processing software.

The UE 16 is a wireless communication device such as a datacommunication device, a voice communication device, a multiple-modecommunication device that supports data, voice, and possibly furthercommunication functions, or a wireless modem that operates inconjunction with a computer system. The UE 16 receives communicationsignals from and preferably also sends communication signals to the BTSs10, 12, 14 through a transceiver or a receiver and a transmitter, and anantenna system that may include a single antenna or multiple antennas.As in the BTSs 10, 12, 14, such signal processing operations asmodulation and demodulation, coding and decoding, filtering,amplification, and frequency conversion are preferably performed by aDSP or general-purpose processor in the UE 16.

Communication signals between BTSs and UEs in a communication networkare formatted according to a particular protocol or communication schemefor which the communication network is adapted. Such signal formats arealso commonly referred to as physical layer structures.

Known physical layer structures for the above example of OFDMcommunications include those used in DAB (Digital Audio Broadcasting)and DVB-T (Digital Video Broadcasting-Terrestrial), and communicationnetworks based on the IEEE 802.11a and 802.16a standards.

DAB and DVB-T are used for audio and video terrestrial broadcasting.Each broadcast station inserts symbol prefixes before each OFDM symboland transmits the same signal as other broadcast stations as asimulcast. Most synchronization methods adopted by DAB and DVB-T systemsare based on the repetition structure introduced by cyclic prefixinsertion. As those skilled in the art will appreciate, prefix-basedinitial access schemes are generally used only when fast network accessis not critical. Normally, DAB and DVB-T are also applied in singlefrequency synchronized networks. In this case, interference fromneighboring broadcast stations is treated as an active echo, which canbe handled by the proper design of the OFDM symbol prefix.

IEEE 802.11a and 802.16a refer to sets of specifications, available fromthe IEEE (Institute of Electrical and Electronics Engineers) relating toOFDM wireless networks. These specifications respectively describepacket-based OFDM systems with and without a preamble header. However,these systems are designed for a single cell, single BTSpoint-to-multi-point transmission in fixed wireless networks.

Another conventional downlink (BTS to UE) point-to-multi-point physicallayer structure for OFDM communication signals includes frames thatcontain time slots, with each time slot containing a number of symbols.FIG. 2 illustrates such a physical layer structure for a synchronizedcommunication network. In this structure, each BTS 10, 12, 14 uses thesame band of frequencies (i.e., frequency re-use is 1), and a datastream from each BTS 10, 12, 14 is organized into time slots which formframes. Each time slot 32, 34, 36, 42, 44, 46, 52, 54, 56 contains apreamble header 30, 40, 50 followed by traffic data symbols. In FIG. 2,three time slots are shown for each of the BTSs 10, 12, 14. For the BTS10, for example, each of the three time slots 32, 34, 36 includes thesame preamble 30 associated with the BTS 10. Similar time slots areshown for the other BTSs 12, 14. In a synchronized network, the framesand time slots and thus the preambles from each BTS are aligned.

FIG. 3 is a time-frequency representation of an example preamblestructure for a synchronized communication network. The preambleincludes training signals transmitted over pilot and synchronizationsub-carriers, and optionally, broadcasting sub-carriers to reduceoverhead. In the structure of FIG. 3, the preamble occupies two symbols58 and 59. Data is transmitted following the preamble, as well as in thebroadcasting sub-carriers, if present. Those skilled in the art will befamiliar with these types of training signals and their use incommunication networks.

The use of a unique orthogonal preamble for each BTS 10, 12, 14 in asynchronized network allows the UE 16 to perform at least fundamentalaccess operations. Orthogonal preambles are generally preferred, toprovide for a fast search operation during initial network access. Aseach BTS has its own unique corresponding preamble, detection of apreamble allows a UE to identify a BTS. In addition, a preamble includesa sequence that is known to UEs, such that preambles further provide forrelatively fast determination of channel quality in terms of a C/I(carrier to interference) ratio, for example, as well as frame andtiming synchronization, frequency and timing offset estimation, andinitial channel estimation, in accordance with known techniques.

FIG. 4 illustrates a physical layer structure for an unsynchronizedcommunication network. As shown, in unsynchronized or asynchronouscommunication networks, the frame boundaries and thus the time slots 62,64, 66 of the BTS 10 do not align with the time slots 68, 70 from theBTS 12 or the time slots 72, 74, 76 from the BTS 14. Preambles insertedinto these time slots according to the techniques described above withreference to FIG. 2 would not be orthogonal, thereby degrading theperformance of preamble-based algorithms. As described above, payloadmanagement associated with preamble insertion may also be incompatiblewith the current UTRAN timing structure, the 3GPP TTI.

In accordance with one embodiment this invention, a new design of aphysical layer structure that is not limited to synchronous networks isprovided. A new common channel provides training information that istypically provided in multiple training channels to enable highperformance initial access and synchronization algorithms whileintroducing very low overhead into communication signals.

FIG. 5 illustrates a physical layer structure in accordance with anembodiment of the invention. Each BTS 10, 12, 14 uses the same frequencyband which preferably includes a plurality of sub-carriers. Although theBTS data streams are separated in FIG. 5 for clarity, it should beappreciated that each BTS 10, 12, 14 preferably uses the same frequencyband.

The time slots 82, 84, 86, 92, 94, 102, 104, 106 preferably include aplurality of symbols, such as OFDM symbols. Each symbol includes a setof time domain samples transferred from a group of frequency domain datathrough a frequency to time domain transformation such as an IFFT, asdescribed above. OFDM symbols may be considered a plurality of signalcomponents, each carried by respective a sub-carrier. Each symbolincludes respective signal components carried by the same plurality ofsub-carriers. Thus any sub-carrier, corresponding to a single frequency,carries a set of time-continuous signal components from a plurality ofsymbols.

A common initial access channel (IACH) 80, 90, 100 is mapped distinctlyto at least one such set of time-continuous signal components, from aplurality of symbols included in the time slots 82, 84, 86, 92, 94, 102,104, 106. FIG. 5 shows a mapping of the IACH 80, 90, 100 to two sets oftime-continuous signal components, although mapping to fewer or furthersets is also contemplated. The IACH is used for one or more of initialaccess, synchronization, base station identification, and channelestimation, as described in further detail below.

It should be appreciated that references to time-continuous signalcomponents are not intended to indicate absolute and perpetual timecontinuity. For example, communication signal processing is typicallyimplemented digitally, using communication signal samples. Furthermore,a set of signal components carrying IACH information may be continuousonly over some of the symbols in a time slot or frame, or preferably anentire frame. The IACH might not be transmitted in certain types offrame or when a BTS is idle (i.e., not transmitting communicationsignals), for instance.

In the case of preambles, training channels are mapped along thefrequency direction in a time-frequency plane, as shown in FIG. 3.However, according to an embodiment of the invention, the IACH 80, 90,100 for each BTS 10, 12, 14 is mapped along the time direction. In apreferred embodiment, the IACH 80, 90, 100 is allocated on the samefrequency or sub-carrier index, which is associated with the frequencyor sub-carrier for the set of time-continuous signal components to whichthe IACH is mapped, for all of the BTSs 10, 12, 14. For example, wherethe BTSs 10, 12, 14 are assigned a frequency band including 10sub-carriers, the IACH 80, 90, 100 may be mapped to the 3^(rd) and7^(th) sub-carriers for each BTS 10, 12, 14. This design introduces verylow overhead, but provides enough training information to support highperformance algorithms.

Advantageously, embodiments of the invention can be optimized forunsynchronized communication networks. In some embodiments, overheadintroduced by the training channels is evenly distributed in each OFDMsymbol. Some embodiments are more robust and involve less complicatedfrequency domain BTS acquisition without the initial rough knowledge offrame boundaries or symbol boundaries.

The initial access channel (IACH) 80, 90, 100 is an initial acquisitionchannel for a mobile terminal such as the UE 16 of FIG. 1 to access acommunication network. For an asynchronous system, several sub-carriershaving the same frequency index are preferably allocated for the IACH. Acommon synchronization code is then preferably mapped onto the IACH. Thecommon synchronization code includes a complex pseudo-noise (PN)sequence that is known to all mobile terminals and used by all BTSs tomodulate the IACH sub-carriers. The same IACH mapping structure ispreferably employed across an entire communication network. A furthercell-specific synchronization code that is unique to each BTS may alsobe mapped onto the IACH. In a preferred embodiment, the common andcell-specific codes are orthogonal and overlap. Each code, by virtue ofits orthogonality with the other code, can then be detected bycorrelation, for example, as described in further detail below.

In order to access a network in which the above IACH and synchronizationcodes are implemented, a mobile terminal first performs a network searchby synchronizing to the common synchronization code to establish thecorrect timing and frame/slot synchronization, and then performs a cellsearch for the cell-specific synchronization code to lock onto a servingBTS.

For OFDM systems, frequency domain acquisition algorithms are generallymore accurate than time domain algorithms because of the embeddedorthogonality property of frequency domain multiplexing. However, inorder to extract the frequency domain sub-carriers, FFT computing orsome other frequency domain transformation is required. Therefore, aninitial acquisition step of coarse timing synchronization is typicallyperformed in the time domain prior to the FFT operation. A cyclic prefixmay be employed to perform the coarse timing acquisition. On the otherhand, the performance of cyclic prefix-based acquisition is poor forshort prefix lengths, and for low C/I ratios. Another issue associatedwith prefix-based acquisition is that frame synchronization cannot berealized jointly with coarse timing acquisition.

The distribution of the IACH along the time direction in accordance withan embodiment of the invention allows terminals to perform framesynchronization and coarse timing synchronization, as well as BTSidentification. Such an IACH can support low complexity frequency domaininitial access acquisition algorithms even without the assistance oftime domain coarse timing synchronization. Initial access methodsemploying the IACH are described in detail below.

In another embodiment of the invention, a scattered pilot channel isalso provided and used in synchronization operations. For OFDM mobilecommunications systems, scattered pilot channels are typically used toestimate the propagation channel to support coherent detection of acommunication signal at a receiver. In this embodiment of the invention,either the common synchronization code or preferably the cell-specificsynchronization code is mapped onto a scattered pilot channel. Mappingof the cell-specific synchronization code to the pilot channeleffectively re-uses the pilot channel for BTS identification, finetiming synchronization, and frequency offset tracking, in addition tochannel estimation.

As described above, only one common synchronization code is used by allBTSs in a communication network, whereas a plurality of uniquecell-specific synchronization code, one per BTS, are used in thenetwork. As the number of the pilot channel sub-carriers is normallygreater than the IACH sub-carriers, the longer cell-specificsynchronization codes, or a repetition of the cell-specificsynchronization codes if necessary, are preferably applied to the pilotchannel.

The positioning of scattered pilot sub-channels is preferably cyclicallyshifted between adjacent BTSs. Power boosted transmission may also beused for the scattered pilot channel. The relationship between a BTS anda scattered pilot sub-carrier pattern is preferably fixed, such thatonce a BTS has been identified, the scattered pilot sub-carrier patternfor that BTS is also known or can be determined by a UE.

In addition to channel estimation, scattered pilot channels are re-usedfor fine timing synchronization and frequency synchronization inembodiments of the present invention. In one embodiment, this isfacilitated by assigning the scattered pilot channel to sub-carriershaving indexes that are the same for pairs of consecutive OFDM symbols.A pair-wise scattered pilot channel sub-carrier pattern not only allowsthe re-use of the pilot channel for frequency synchronization or offsettracking, but may also supports MIMO (Multiple Input Multiple Output)systems. Re-use of the pilot channel for frequency synchronization oroffset tracking reduces communication signal overhead, in thatconventional techniques employ a plurality of training channels forchannel estimation and synchronization in time and frequency.

FIG. 6 shows an initial access channel and scattered pilot channelsub-carrier pattern according to an embodiment of the invention, for anOFDM communication network. Each row represents an OFDM symbol, and eachcolumn represents a set of sub-carriers having a common frequency index.Thus, each circle in FIG. 6 represents a signal component, carried by aparticular sub-carrier, of a symbol.

In the illustrative example of FIG. 6, the pilot channel 110 ispair-wise scattered onto sub-carriers having the same index in pairs ofconsecutive symbols. The pilot channel sub-carrier pair 110 a includessub-carriers that have the same frequency index in the consecutivesymbols 111 a and 111 b. Similarly, the pilot channel sub-carrier pair110 b includes sub-carriers that have another common index in the nextconsecutive pair of symbols. The IACH is mapped to multiple sets 112,114 of time-continuous signal components from a plurality of symbols.

The spacing between IACH sub-carriers and the spacing between pilotsub-carriers is also chosen as a power of 2 in the example embodiment ofFIG. 6. This spacing enables application of a faster pilot channelextraction algorithm, for example by performing partial FFTs.

The combination of the common synchronization code and the cell-specificsynchronization code may be considered a synchronization sequence. Thetwo codes of the synchronization sequence may be mapped to the IACH, andone of the codes, preferably the cell-specific synchronization code, mayalso be mapped to the scattered pilot channel, as described above. Inone embodiment of the invention, the common synchronization code ismapped to the IACH, whereas the cell-specific code is mapped to the IACHas well as the scattered pilot channel. It will be obvious to thoseskilled in the art that this is one example code-to-channel mapping, andthat the invention is in no way limited thereto.

The present invention is similarly not limited to synchronizationchannel information having two constituent codes or types of codes. Forexample, the techniques described herein for mapping commonsynchronization codes and cell-specific synchronization codes onto theIACH and scattered pilot channel may be extended to other types ofsynchronization channel information, such as 3GPP synchronizationchannel information. In accordance with embodiments of the invention,such a synchronization channel is mapped onto a physical layer structurefor asynchronous communication networks.

In the context of the 3GPP synchronization channel, 3GPP PSCs (primarysynchronization codes) and SSCs (secondary synchronization codes), whichmay be used by more than one BTS, are forms of the commonsynchronization code. As each BTS has one and only one unique primaryscrambling code, the scrambling code is a form of the cell-specificsynchronization code. The PSC is used to acquire slot timing, the SSC isused to acquire frame timing, and the scrambling code is used to acquirea connection with a particular BTS. The generation of 3GPP scramblingcodes and synchronization codes is known and is therefore described onlybriefly herein.

A total of 2¹⁸−1=262,143 scrambling codes, numbered 0 . . . 262,142 canbe generated using 18^(th) order generator polynomials, for example.However, not all of these scrambling codes are used according to 3GPPspecifications. The scrambling codes are divided into 512 sets of 15secondary scrambling codes, with each set corresponding to one of 512primary scrambling codes. Each BTS is allocated one and only one primaryscrambling code.

Scrambling codes are constructed by combining two real sequences into acomplex sequence. Each of the two real sequences is constructed as theposition-wise modulo 2 sum of 38400 chip segments of two binarym-sequences generated by means of two generator polynomials of degree18. The resulting sequences thus constitute segments of a set of Goldsequences. The scrambling codes are repeated for every 10 ms radioframe. If x and y are the two sequences, the x sequence is preferablyconstructed using the primitive (over GF(2)) polynomial 1+X⁷−X¹⁸ and they sequence is preferably constructed using the polynomial1+X⁵+X⁷+X¹⁰+X¹⁸.

The primary scrambling codes include scrambling codes n=16*i where i=0 .. . 511. The i^(th) set of secondary scrambling codes consists ofscrambling codes 16*i+k, where k=1 . . . 15. Each primary scramblingcode is thus associated with 15 secondary scrambling codes, such thatthe i^(th) primary scrambling code corresponds to the i^(th) set ofsecondary scrambling codes.

Hence, according to the above, scrambling codes 0, 1, . . . , 8191 aretypically used. Each of these codes is associated with a leftalternative scrambling code and a right alternative scrambling code thatmay be used for compressed frames. The left alternative scrambling codecorresponding to scrambling code k is scrambling code number k+8192,while the right alternative scrambling code corresponding to scramblingcode k is scrambling code number k+16384. For compressed frames, theleft alternative scrambling code is used if n<SF/2 and the rightalternative scrambling code is used if n≧SF/2, where c_(ch,SF,n) is thechannelization code used for non-compressed frames. The usage ofalternative scrambling codes for compressed frames is respectivelysignalled by higher layers for each physical channel.

The set of 512 primary scrambling codes is further divided into 64scrambling code groups, each consisting of 8 primary scrambling codes.The j^(th) scrambling code group consists of primary scrambling codes16*8*j+16*l, where j=0 . . . 63 and l=0.7.

Each BTS is allocated one and only one primary scrambling code. TheP-CCPCH (Primary Common Control Physical Channel), primary CPICH (CommonPilot Channel), PICH (Page Indication Channel), AICH (AcquisitionIndication Channel), AP-AICH (Access Preamble AICH), CD/CA-ICH(Collision Detection/Channel Assignment Indication Channel), CSICH(Common Packet Channel (CPCH) Status Indicator Channel) and S-CCPCH(Secondary CCPCH), for example, are transmitted using the primaryscrambling code. Other downlink physical channels can be transmittedwith either the primary scrambling code or a secondary scrambling codefrom the set associated with the primary scrambling code of the BTS. Amixture of primary scrambling code and secondary scrambling code for oneCCTrCH (Coded Composite Transport Channel) is allowable. However, in thecase of the CCTrCH of type DSCH (Downlink Shared Channel), all the PDSCH(Physical DSCH) channelization codes that a single terminal may receiveshall then be under a single scrambling code, either the primary or asecondary scrambling code.

The scrambling code associated with a scrambling code number n isdenoted z_(n). When x(i), y(i) and z_(n)(i) denote the i^(th) symbols ofthe sequences x, y, and z_(n), respectively, the m-sequences x and y areconstructed using the following initial conditions:

x is constructed with x(0)=1, x(1)=x(2)= . . . =x(16)=x(17)=0;

y(0)=y(1)= . . . =y(16)=y(17)=1, and

a recursive definition of subsequent symbols as:

x(i+18)=x(i+7)+x(i) modulo 2, i=0, . . . , 2¹⁸−20;

y(i+18)=y(i+10)+y(i+7)+y(i+5)+y(i) modulo 2, i=0, . . . , 2¹⁸−20.

The n^(th) Gold code sequence z_(n), n=0, 1, 2, . . . , 2¹⁸−2, is thendefined as:

z_(n)(i)=x((i+n) modulo (2¹⁸−1))+y(i) modulo 2, i=0, . . . , 2¹⁸−2.

These binary sequences are then converted to real valued sequences Z_(n)by the following transformation:

${Z_{n}(i)} = \left\{ {{{\begin{matrix}{+ 1} & {{{if}\mspace{14mu} {z_{n}(i)}} = 0} \\{- 1} & {{{if}\mspace{14mu} {z_{n}(i)}} = 1}\end{matrix}\mspace{14mu} {for}\mspace{14mu} i} = 0},1,\ldots \mspace{14mu},{2^{18} - 2.}} \right.$

Finally, the n^(th) complex scrambling code sequence S_(dl,n) is definedas:

S_(dl,n)(i)=Z_(n)(i)+jZ_(n)((i+131072) modulo (2¹⁸−1)), i=0, 1, . . . ,38399.

Note that the pattern from phase 0 up to the phase of 38399 is repeated.

Turning now to synchronization codes, the PSC, also denoted C_(psc), isconstructed as a so-called generalized hierarchical Golay sequence. ThePSC is furthermore chosen to have good aperiodic autocorrelationproperties, which allow for detection of the PSC at a receiver.

$\begin{matrix}{{{{Let}\mspace{14mu} a} = {< x_{1}}},x_{2},x_{3},\ldots \mspace{14mu},{x_{16} >}} \\{{= {< 1}},1,1,1,1,1,{- 1},{- 1},1,{- 1},1,{- 1},1,{- 1},{- 1},{1 > .}}\end{matrix}$

The PSC is generated by repeating the sequence a modulated by a Golaycomplementary sequence, and creating a complex-valued sequence withidentical real and imaginary components. The PSC C_(psc) is defined as:

C_(psc)=(1+j)×<a, a, a, −a, −a, a, −a, −a, a, a, a , −a, a, −a, a, a>,

where the leftmost chip in the sequence corresponds to the chiptransmitted first in time.

Each code in a set of 16 SSCs {C_(ssc,1), . . . , C_(ssc,16)}, is alsocomplex-valued with identical real and imaginary components, and areconstructed from position-wise multiplication of a Hadamard sequence anda sequence d, defined as:

d=<b, b, b, −b, b, b, −b, −b, b, −b, b, −b, −b, −b, −b, −b>,whereb=<x₁, x₂, x₃, x₄, x₅, x₆, x₇, x₈, −x₉, −x₁₀, −x₁₁, −x₁₂, −x₁₃, −x₁₄,−x₁₅, −x₁₆> andx₁, x₂, . . . , x₁₅, x₁₆, are the same as in the sequence a above.

The Hadamard sequences are obtained as the rows in a matrix H₈constructed recursively by:

$\begin{matrix}{H_{0} = (1)} \\{{H_{k} = \begin{pmatrix}H_{k - 1} & H_{k - 1} \\H_{k - 1} & {- H_{k - 1}}\end{pmatrix}},{k \geq 1},}\end{matrix}$

with the rows numbered from the top starting with row 0 (the all onessequence).

Denoting the n^(th) Hadamard sequence as a row of H₈ numbered from thetop, n=0, 1, 2, . . . , 255, and h_(n)(i) and d(i) denote the i^(th)symbols of the sequences h_(n) and d, respectively, where i=0, 1, 2, . .. , 255 and i=0 corresponds to the leftmost symbol, the k^(th) SSC,C_(ssc,k), k=1, 2, 3, . . . , 16 is then defined as:

C _(ssc,k)=(1+j)×<h _(m)(0)×d(0), h _(m)(1)×d(1), h _(m)(2)×d(2), . . ., h _(m)(255)×d(255)>,

where m=16×(k−1) and the leftmost chip in the sequence corresponds tothe chip transmitted first in time.

In 3GPP, a 10 ms radio frame includes 15 time slots. The PSC and an SSCare transmitted in parallel in each time slot. Although the PSC is thesame for every BTS in a communication network and is transmitted inevery time slot, the SSCs in each time slot need not be the same. A setof 64 sequences of 15 of the 16 SSCs is constructed such that theircyclic shifts are unique, i.e., a non-zero cyclic shift less than 15 ofany of the 64 sequences is not equivalent to some cyclic shift of anyother of the 64 sequences. Also, a non-zero cyclic shift less than 15 ofany one of the sequences is not equivalent to itself with any othercyclic shift less than 15. The 64 sequences, also commonly referred toas secondary synchronization channel (SCE) sequences, are uniquelyassociated with one of the 64 groups of primary scrambling codes. The 15SSCs of a secondary SCE sequence are respectively transmitted in the 15time slots of the radio frame.

FIGS. 7( a) and 7(b) together form a table showing an example allocationof SSCs for secondary SCE sequences to encode the 64 differentscrambling code groups. The entries in the table denote which SSCs touse in the different slots depending upon the scrambling code group towhich the primary scrambling code of a BTS belongs. For example, theentry “7” means that SSC C_(ssc,7) shall be used for the correspondingscrambling code group and slot, such as in slots 11 and 13 when theprimary scrambling code belongs to scrambling code group 0.

Although 3GPP synchronization codes and scrambling codes are known tothose skilled in the art to which the present invention pertains, themapping of such codes for unsynchronized communication networks inaccordance with embodiments of the invention is novel. It should also beappreciated that the above represents an example of code generation. Thepresent invention is in no way limited thereto.

FIGS. 8 and 9 are block diagrams showing examples of mappings between asynchronization channel and OFDM symbols in accordance with embodimentsof the invention. In FIGS. 8 and 9, 3GPP synchronization channels aremapped onto the logical synchronization channel structure and physicallayer structure described above.

In the mapping of FIG. 8, the PSC 128 and the SSC 130 are mapped to thecommon synchronization code 124 of the logical synchronization channel122, and to the IACH 134. Referring to FIG. 6, the PSC 128 and the SSC130 may be mapped to respective components 112 and 114 of the IACH, forexample. The scrambling code 132, which may be a primary scrambling codeor a secondary scrambling code, for example, is mapped to thecell-specific synchronization code 126, and a portion of the scramblingcode 132 is mapped to each of the IACH 134 and the scattered pilotchannel 136. Alternatively, the entire scrambling code 132 could bemapped to the scattered pilot channel 136. The overhead of the IACH inthe mapping shown in FIG. 8 is approximately 8-10%.

A further alternative mapping is shown in FIG. 9, in which the logicalchannel mapping of the PSC 148, the SSC 150, and the scrambling code 152to the common synchronization code 144 and the cell-specificsynchronization code 146 of the synchronization channel 142 is similarto that of FIG. 8, although mapping to the physical layer is different.In FIG. 9, the PSC 148 is mapped to the IACH 154, whereas the SSC 150and the scrambling code 152 are mapped to the scattered pilot channel156. In this case the IACH overhead is roughly 4%.

Optimization of network acquisition performance, i.e. acquisition timeand success rate, trades off the overhead introduced by the IACH.Generally, acquisition performance improves as overhead is increased.

One example of an initial access method based on the physical layerstructure proposed herein is illustrated in FIG. 10. In the method 160,the first task 162 is to perform a search for a common synchronizationcode to achieve correct frame and slot timing.

According to one embodiment of the invention adapted for OFDMcommunications, a time domain to frequency domain transformation,illustratively an FFT, is performed on a received communication signalat a start position or sample index. In a multiple-BTS network, thereceived communication signal may include signals transmitted fromdifferent BTSs. The FFT start position is preferably determined from aninitial coarse timing estimate, which may be generated using a timedomain algorithm such as a prefix-based time domain estimationalgorithm, for instance.

IACH sub-carriers from within a window preferably having a length of oneframe are then sampled, in the frequency domain, to extract an IACHvector. For 3GPP, a radio frame has a length of 10 ms. As describedabove, the common synchronization code for a communication network is aPN sequence that is known to communication terminals configured tooperate within the network. The extracted IACH vector is correlated withthe known PN sequence corresponding to the common synchronization code.

The frame window is then moved a distance of one OFDM symbol at a time,the IACH sub-carriers within the relocated frame window are extracted,and the extracted IACH vector is correlated with the commonsynchronization code. This process of moving the frame window,extracting the IACH sub-carriers, and correlating the extracted IACHvector with the common synchronization code is preferably repeated untilthe frame window has scanned all of the OFDM symbols within one frameperiod.

Peaks in the resultant correlation pattern, or the locations of peakswith correlation values above a predetermined threshold, within oneframe period are indicative of correct alignment of the frame window fordetection of the common synchronization code from one of a plurality ofBTSs. OFDM symbols at positions corresponding to these peaks arecandidate first OFDM symbols in frames transmitted from the strongestadjacent BTSs. In one embodiment of the invention, frame windowpositions and correlation values are tracked using OFDM symbol offsetindexes, and the offset indexes associated with the correlation peaksare then used to identify the corresponding first OFDM symbols.

The above common synchronization code search scheme uses an initialcoarse timing estimate for FFT calculation. According to a furtherembodiment of the invention, timing and frame synchronization areperformed jointly, such that no time domain coarse timing estimate isused.

The function of the coarse timing estimation is to roughly find theranges of the starting positions of OFDM symbols for the strongest BTSs,and therefore to provide FFT window positions for subsequent BTSidentification as described in further detail below, as well as furthertiming and frequency offset estimation. For joint frame and coarsetiming synchronization, the frame boundaries and the coarse timingsynchronization positions are obtained at the same time.

As in the preceding common synchronization code search scheme, an FFT isperformed on a received communication signal, and IACH vectors areextracted from IACH sub-carriers for each of a plurality of positions ofa frame window and correlated with the known common synchronizationcode. Where no initial timing estimate is determined however, thestarting position of the FFT window is moved within one OFDM symbol, andfor each position of the FFT window, the extraction, correlation, andframe window moving operations are repeated. In one embodiment, the FFTwindow starting position is moved one sample at a time until thecumulative distance from an initial starting window position is equal tothe length of one OFDM symbol. To save processing power and speed up thesearching procedure, the FFT window may instead be moved in N-samplesteps, where N is a predetermined parameter. According to a preferredembodiment, N is equal to the length of a cyclic prefix of an OFDMsignal.

During this two-dimensional search process in which the FFT window andthe frame window are moved, frame boundaries can be found by identifyingthe OFDM symbols, or indexes thereof, related to the correlation peaks.Coarse timing synchronization positions can be obtained through furthersearching FFT window positions in the vicinity of each individual peak,using procedures substantially similar to those described above but witha smaller step size, to locate FFT window positions corresponding tolocal maximum correlation values for each correlation peak.

Thus, frame synchronization is based on detection of the commonsynchronization code, and coarse timing synchronization may be eitherbased on an initial coarse timing estimate or determined jointly withframe synchronization.

After the frame synchronization and the coarse timing synchronization, acommunication terminal proceeds to a cell-specific synchronization codesearch at 164. It will be apparent from the foregoing that where acommunication signal received by a terminal includes signals transmittedfrom multiple BTSs, the terminal obtains several pairs of thesynchronization parameters, one pair per BTS. The synchronizationparameters include candidate timing positions, as FFT window positions,and candidate frame boundaries. To find the serving BTS, which is theBTS from which the strongest communication signal is received, andactive neighbour BTSs for possible BTS selection and handoff, BTSidentification operation is performed. This can be done based on thecell-specific synchronization codes carried by IACH.

In one embodiment of the cell-specific code search at 164, for eachcandidate OFDM symbol or index indicating the beginning of a frame, asdetermined from the frame synchronization process, an FFT is performedwith using a corresponding FFT window determined from the coarse timingsynchronization process. Data from the IACH is extracted and correlatedwith cell-specific synchronization codes of all known BTSs with which aterminal may communicate. As will be apparent to those skilled in theart, a communication terminal is provided with cell-specificsynchronization codes for BTSs in a communication network during networkregistration or activation procedures. The extracted IACH data iscorrelated with each of these codes or a subset of these codes. Acorrelation peak indicates the detection of one of the cell-specificsynchronization codes. BTS indexes, which are either used as thecell-specific synchronization codes or uniquely associated with thecell-specific synchronization codes, corresponding to the correlationpeaks for respective correlation patterns for each of the candidateframe start positions and the related coarse synchronization positionsare then determined.

Signal strength measures such as C/I ratio may be calculated for eachidentified BTS on the basis of a detected cell-specific synchronizationcode, a common synchronization code, or some combination thereof. Theserving BTS is the strongest BTS and other candidate BTSs are the BTS onthe active neighbour BTS list.

When the above joint frame synchronization and coarse timingsynchronization procedure is used, then the coarse timing positionsdetermined from subsequent searching of the maximum correlation peakpositions are used as the FFT window starting positions in thecell-specific synchronization code search for each of the candidatefirst frame symbols. If an initial coarse timing estimate was made todetermine a starting position for the FFT window used for commonsynchronization code searching at 164, then the same FFT window positionwould be used for during cell-specific synchronization code searching.In this case, since the same FFT has already been performed, arepetition of the FFT for the cell-specific search may be avoided. Forexample, the FFT results may be stored in memory during the commonsynchronization code search for subsequent retrieval duringcell-specific synchronization code searching. In another embodiment,data extracted from the IACH is stored. According to a furtherembodiment, data is extracted from both the IACH and the scattered pilotchannel during common synchronization code searching and stored inmemory. The storage of FFT results or extracted data reduces the amountof processing required during cell-specific synchronization codesearching. Memory access operations are generally faster and lessprocessor-intensive than performing FFTs or other transformations.

To correctly recover communication signals from a serving BTS, acommunication terminal further synchronizes to the serving BTS both intime and in frequency at 166. Fine timing synchronization and frequencyoffset estimation are preferably achieved using the scattered pilotchannel. Although a synchronization code may be mapped to the scatteredpilot channel in accordance with embodiments of the invention,implementations of the present invention preferably do not preclude theuse of the pilot channel for such further synchronization of a terminalto a BTS. Thus, embodiments of the present invention re-use pilotchannels for initial access operations but preferably do not precludeconventional uses of such channels for other operations, includingchannel estimation and fine synchronization.

FIG. 11 is a flow chart showing an initial access method according toanother embodiment of the invention adapted for 3GPP synchronizationcodes. The illustrative example method of FIG. 11 is compatible withstandard 3GPP operations with respect to the PSC, SSC, and scramblingcode. Such a backward-compatible initial access scheme allows re-use ofthe CDMA (Code Division Multiple Access) search engine of 3GPP terminalsto perform the cell search for asynchronous OFDM communication networks.

The method 170 begins at 172 when a cell search operation is initiated.A cell search may be initiated by a user of a communication terminal orby the terminal itself, for example at predetermined intervals or whenreceived signal strength or quality between the terminal and a currentBTS degrades to a predetermined degree.

In the method 170, common synchronization code searching includessearching for the PSC, or a portion thereof, which is common to all BTSsin a communication network, at 174, as well as searching for the SSC at176. PSC detection at 174 is normally accomplished using a matchedfilter or a similar device that is matched to the known PSC for thecommunication network. Scrambling code group detection at 176 involvesdetection of a sequence of SSCs that is associated with a particularscrambling code group. SSC detection is preferably performedsubstantially as described above, by correlating data extracted from theIACH with the SSCs corresponding to the detected PSC. As describedabove, an SCE sequence of 15 of the 16 SSCs associated with a PSC, whichare also known or can be determined by the terminal, identifies thescrambling code group to which the primary scrambling code used by a BTSbelongs. SCE sequence detection is preferably performed by correlationof extracted IACH data with each of the 64 SCE sequences associated withthe PSC, but may instead be performed on an SSC-by-SSC basis.

The method 170 then proceeds to 177 to determine whether framesynchronization has been achieved. The determination at 177 involvesmaking a determination as to whether any candidate frame start positionshave been identified. If not, then the method 170 reverts back to thecommon code search operation. When one or more candidates have beenidentified, the cell-specific synchronization code search operations at178-182 are performed for each candidate. At 178, after the scramblingcode group has been determined at 176, data is extracted from the IACHand correlated with each of the primary scrambling codes in thescrambling code group. The primary scrambling code corresponding to thelargest correlation peak in the resultant correlation patterns and thusthe strongest BTS is selected at 180 and used, and the search procedureends at 184.

Although not explicitly shown in FIG. 11, it should be appreciated thatthe operations in FIG. 11 may be repeated, such as to ensure that aselected BTS is still the strongest BTS or when subsequent communicationsignals received from the selected BTS cannot be properly decoded. Thismay involve repeating 178 and 180 for previously identified candidateBTSs, or repeating the entire search procedure to identify new candidateBTSs.

FIG. 12 is a block diagram of a communication terminal in which thepresent invention may be implemented. The communication terminal 191includes an antenna 190 connected to a common synchronization codedetector 192, which is connected to a cell-specific synchronization codedetector 194. Both code detectors 192, 194 are connected to a memory196.

The antenna 190, although shown as a single antenna, may includemultiple antenna elements, to provide receive diversity, for example.

The common synchronization code detector 192 and the cell-specificsynchronization code detector 194 detect synchronization codes inreceived signals substantially as described above. The detectors 192,194 are preferably implemented in software code, such as a moduleexecutable by a processor (not shown) in the communication terminal 191.In a preferred embodiment, both detectors 192, 194 are implemented in aDSP 198, which may also support other signal processing functions of thecommunication terminal 191. Thus, a communication terminal 191 accordingto an embodiment of the invention includes an input for receivingcommunication signals and a processor for processing received signals.

Data for synchronization code detection, including a commonsynchronization code and at least one cell-specific synchronizationcode, is stored in the memory 196. The memory 196, preferably a solidstate memory component, may also store data associated with other signalprocessing functions or other components of the communication terminal191. In some communication terminals, the memory 196 is removable, forexample as part of a computer card that enables the terminal for networkcommunications. For the purposes of synchronization code detection, thedetectors 192, 194 read information from the memory 196. However, thedetectors 192, and other components of the communication terminal 191may also read and preferably write data to the memory 196. A writeablememory 196 supports such functions as over-the-air communication networkinformation updates.

The operation of the communication terminal 191 in accordance withembodiments of the invention will be apparent from the foregoingdescription. The common synchronization code detector 192 receivescommunication signals from one or more BTSs via the antenna 190 andsearches for a known common synchronization code that was previouslystored to the memory 196 and is retrieved from the memory 196 during acell search operation. A coarse timing estimator may or may not beprovided in the detector 192, as coarse timing estimation may beperformed jointly with frame synchronization. Where a coarse timingestimator is provided, implementation of the memory 196 in a writeablestore allows the detector 192 to store FFT results and possibly dataextracted from the IACH to the memory 196. The common synchronizationcode detector 192 outputs pairs of synchronization parameters, includingcandidate first frame symbols and candidate FFT window positions, to thecell-specific synchronization code detector 194 and/or the memory 196.

The cell-specific synchronization code detector 194 similarly searches areceived communication signal for one or more cell-specificsynchronization codes, based on the candidate first frame symbols andcoarse timing estimates from the common synchronization code detector192. In FIG. 12, the cell-specific synchronization code detector 194 isconfigured to receive both synchronization parameters and a receivedcommunication signal through the common synchronization code detector192. In other embodiments, the cell-specific synchronization codedetector 194 is connected to the antenna 190 and thus receivessynchronization parameters from the common synchronization code detector192 and receives a communication signal from the antenna 190.

As described above, repetition of FFT and possibly data extractionoperations may be avoided, particularly where an initial coarse timingestimator is provided, by storing FFT results and data extracted fromthe IACH to the memory 196 during common synchronization code searching.The cell-specific synchronization code detector 194 then retrieves FFTresults and/or extracted data from the memory 196.

Those skilled in the art will appreciate that only components that areinvolved in initial access operations according to embodiments of theinvention have been shown in the communication terminal 191. Acommunication terminal normally includes further components forsupporting other functions. The present invention is in no way limitedto communication terminals that include only those elements shown inFIG. 12.

FIG. 13 is a block diagram of a synchronization code detector. Althougheach of the code detectors 192, 194 is configured to search for adifferent synchronization code retrieved from memory, the code searchingalgorithms preferably include sampling, transformation, and correlationoperations performed by the sampler 200, the FFT element 202, and thecorrelator 204. In a preferred embodiment, the functional elements 200,202, 204 are implemented at least partially in software code that isinvoked during both common synchronization code searching andcell-specific synchronization code searching. Hardware may similarly beshared between the detectors 192, 194.

At a BTS side, a system supporting the physical layer structure andinitial access schemes according to embodiments of the invention isanalogous to the communication terminal 191. The common andcell-specific synchronization codes detected by a communication terminalare mapped onto components of signals to be transmitted by a BTS bymodulating the signal components or carriers thereof, sub-carriers forOFDM-based networks, using the synchronization codes. Thesynchronization codes are stored in a memory and retrieved from thememory when needed. The BTS may include one or more antennas fortransmitting communication signals carrying the IACH and possibly pilotchannels.

FIGS. 14 and 15 illustrate example joint frame and coarse timingsynchronization simulation results. The simulation conditions from whichthe plots shown in FIGS. 14 and 15 were generated are as follows:

-   -   ITU-VA (International Telecommunication Union-Vehicular A)        channel    -   Communication terminal speed: 30 km/h    -   C/I=−3.24 dB    -   Eight BTSs    -   60 OFDM symbols per 10 ms frame    -   OFDM indexes for the first OFDM symbols in the frames from each        BTS, respectively: 14, 12, 8, 22, 27, 17, 2, 32    -   Location of the first sample following the end of the prefix of        an OFDM symbol for each BTS, respectively: 512 10 120 300 350 80        260 500    -   Coarse synchronization search step (=prefix length): 32 samples.

The above conditions were chosen solely as an illustrative embodiment ofthe invention for the purposes of simulation. As such, the invention isin no way limited to this particular embodiment.

It can be seen from FIG. 14 that the first OFDM symbol positions for thefour strongest BTSs, corresponding to symbol indexes 8, 12, 14, and 22,can be identified based on correlation peaks. For FIG. 15, the thirdBTS, which is the strongest BTS having a first symbol position of 8 anda first sample location of 120, is used as the example. The maximumcorrelation value is close to the 161st sample (32*5+1), which is withinone coarse synchronization search step (32 samples) of the first samplelocation of 120+32 sample prefix=152. Subsequent searching of the peakposition with a smaller step size would improve the accuracy of thedetermination of first sample location.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

For example, although described primarily in the context of OFDM, theinvention may be implemented in communication networks that use othertypes of communication signals that include multiple signal components.In accordance with an embodiment of the invention, the IACH is mapped toone or more sets of time-continuous signal components having a commonfrequency.

The invention is similarly not limited to any particular type ofsynchronization channel. 3GPP synchronization codes and scrambling codesare described above solely for illustrative purposes.

In addition, it should be appreciated that the invention may beimplemented in communication networks in which BTSs, communicationterminals, or both, include multiple antennas. Such networks include,but are not limited to, MIMO, MIMO-BLAST, STTD (Space Time TransmitDiversity), and SFTD (Space Frequency Transmit Diversity) networks.

1. A method of transmitting information in an unsynchronized OrthogonalFrequency Division Multiplexing (OFDM) communication network comprisinga plurality of base stations, the method comprising: modulating accesschannel information onto a predetermined initial access channel of anOFDM communications signal, wherein the initial access channel comprisesa predetermined set of one or more time-continuous signal components ofthe OFDM communications signal, each time-continuous signal componentbeing carried by a respective sub-carrier; and transmitting thecommunications signal.
 2. The method as claimed in claim 1, wherein theaccess channel information comprises a common synchronisation code. 3.The method as claimed in claim 2, wherein the common synchronisationcode comprises a complex PN (pseudo noise) sequence associated with aplurality of transceiver stations in a communications network and knownto communication terminals configured for accessing the communicationnetwork.
 4. The method as claimed in claim 1, wherein the initial accesschannel further comprises a cell-specific synchronisation code uniquelyassociated with one of the plurality of transceiver stations.
 5. Themethod as claimed in claim 4, wherein the common synchronisation codecomprises a primary synchronisation code, wherein the cell-specificsynchronisation code comprises a scrambling code associated with asecondary synchronisation code.
 6. The method as claimed in claim 4,wherein the at least one set of time-continuous signal componentscomprises a first set of time-continuous signal components having afirst common frequency and a second set of time-continuous signalcomponents having a second common frequency, and wherein modulatingcomprises modulating the common synchronisation code and thecell-specific synchronisation code to both the first set oftime-continuous signal components and the second set of time-continuoussignal components.
 7. The method as claimed in claim 1, wherein thecommunication channel further comprises a scattered pilot channel, andwherein modulating comprises modulating a first portion of the accesschannel information to the at least one set of time-continuous signalcomponents and modulating a second portion of the access channelinformation to both the at least one set of time-continuous signalcomponents and the scattered pilot channel
 8. The method of claim 1,wherein the set of signal components carrying IACH information iscontinuous only over some of the symbols in a time slot or a frame.
 9. Amethod of accessing a communication network comprising: receiving acommunication signal having a plurality of sets of time-continuoussignal components, the communication signal being an OFDM (OrthogonalFrequency Division Multiplexing) signal; searching for access channelinformation in at least one predetermined set of the plurality of setsof time-continuous signal components, each of the at least one set oftime-continuous signal components comprising signal components carriedby a respective sub-carrier in a plurality of OFDM symbols determiningsynchronisation parameters based on a location of the access channelinformation in the at least one predetermined set of time-continuoussignal components.
 10. The method as claimed in claim 9, whereinsearching comprises searching for a common synchronisation codeassociated with a plurality of base station transceiver stations in thecommunications network and searching for any of the plurality ofcell-specific synchronisation codes respectively uniquely associatedwith the plurality of base transceiver stations.
 11. The method asclaimed in claim 12, wherein searching for the common synchronisationcode comprises: sampling the received communication signal; performing atime domain to frequency domain transformation using a transformationwindow starting at a start position to generate a frequency domainsignal; extracting frequency domain data corresponding to the at leastone predetermined stet of time-continuous signal components from thefrequency domain signal within a window having a length of apredetermined period; correlating the extracted data with the commonsynchronisation code; moving the predetermined period-length window by apredetermined step size until a starting position of the predeterminedperiod-length window has been moved a distance of at least thepredetermined period; repeating the extracting and correlating for eachposition of the predetermined period-length window; and determining peakcorrelation values indicating occurrences of the common synchronisationcode.
 12. The method of claim 11, wherein the communication signalcomprises a plurality of frames, each frame comprising a plurality ofsymbols, wherein the predetermined period is a length of each of theframes, and wherein the step size is a length of each of the symbols.13. The method of claim 9, wherein searching for the access channelinformation comprises searching for a primary synchronisation code, or aportion thereof, which is common to a plurality of basestations in anetwork, as well as searching for a secondary synchronisation code. 14.The method of claim 13, wherein the secondary synchronisation code (SSC)is associated with a scrambling code group.
 15. The method of claim 13,wherein a secondary synchronisation code is detected by correlating dataextracted from an initial access channel (IACH) including the secondarysynchronisation code corresponding to the detected primarysynchronisation code.
 16. The method of claim 15, wherein the extracteddata are a scrambling code group to which the primary scrambling codeused by a basestation belongs.
 17. The method of claim 15, wherein saiddetection is performed on an SSC-by-SSC basis.
 18. The method as claimedin claim 13 or 16, wherein after determining of the scrambling codegroup, data is extracted from the IACH and correlated with each of theprimary scrambling codes in the scrambling code group.
 19. The method asclaimed in claim 13 or 16, wherein the scrambling code is used toacquire a connection with a particular basestation.
 20. A method ofaccessing a communication network comprising: receiving a communicationsignal having a plurality of sets of time-continuous signal components,the communication signal being an OFDM (Orthogonal Frequency DivisionMultiplexing) signal; searching for access channel information in atleast one predetermined set of the plurality of sets of time-continuoussignal components, the access channel information comprising a primarysynchronisation code and a secondary synchronisation code associatedwith a particular scrambling code group to which the primary scramblingcode used by a basestation belongs; and selecting a basestation bydetermining the scrambling code group from the access channelinformation and correlating data from the set of time-continuous signalcomponents with primary scrambling codes of the scrambling code group.